I always understood "favoriting" to be like "saving" a post in reddit. However, the digestabilibty of tweets are pretty much instantaneous so "saving" them is without merit. So I understood the genesis of "hearting" or "liking" to be very much so a re-branding of a highly adopted UI. Was this move organized before @jack came back to Twitter?
That's assuming the tweet is only text. I had used it to quickly bookmark links within tweets, but opening them and saving them to Instapaper honestly works better.
I actually come back to a lot of saved Reddit posts, particularly ones on programming subreddits where I can't easily read the article or mess with the code on my phone.
That's the point - a Reddit post can be much more substantial than a tweet usually is, so saving makes more sense for the former than the latter.
Well, that, and the fact that you have a huge number of people that are going to try it for a bit, and then forget about it. I'd like to see the numbers over the next six months.
People are already 'liking' things on Facebook and Instagram so it isn't as if people are unfamiliar and will forget to use it. Some people who used it as a bookmarking took might not use it anymore, but everyone else can now just transport the mentality of liking things to Twitter too.
You can't A/B test something familiar versus something novel.